A 2005 Gulli & Signorini study showed that the number of web pages that can be indexed is nearing 12 billion. In light of the large number of web pages, web page indexing is used by many we-based applications such as web search engines and web-based advertising applications. Indexing is also generally performed by web spiders and is used by search engines such as GOOGLE and YAHOO! SEARCH. A web spider, or web crawler, is a program or automated script that methodically scans the World Wide Web and collects copies of web pages to be indexed by a search engine.